In a conventional Long Term Evolution (LTE) deployment a number of network nodes, in particular eNodeBs (E-UTRAN Node B or evolved Node B, also denoted eNB), are deployed to provide coverage in a certain area. Each eNodeB can manage one or more cells and also manage all devices, e.g. wireless user equipment, residing within the coverage area of those cells.
From the perspective of the wireless device, the different cells are distinguished by a physical-layer cell identity (PCI) which is defined in 3GPP specification TS 36.211, ch 6.11. In a typical deployment scenario, neighboring cells have different PCIs and when the wireless devices are in a connected state they use these PCIs as an identifier for handover measurements.
Cell merging, also denoted shared cell, combined cell or multi-sector cell in some cases, is a new cell configuration for LTE and enables a multi Radio Resource Unit (RRU) deployment without needing to care about cell planning from a Radio Frequency (RF) reuse perspective; the same radio frequencies are used by all RRUs. The shared cell is achieved by allowing the different RRUs to use the same PCI and thus the wireless device considers all RRUs to be part of the same cell. The spatially separated RRU or a group of RRUs are called sector (or sector carriers). The cell can contain multiple sectors, and a wireless device can belong to one sector or multiple sectors depending on the degree of sector isolation. Insufficient isolation between sectors that are identified identically, as is the case when reusing the PCI, can cause interference and thus reduce communication quality.
FIGS. 1a and 1b illustrate examples of the concepts of different cells (FIG. 1a) and shared cell (FIG. 1b), respectively, used in heterogeneous-network deployment. These cell concepts have been seen as solutions for providing higher data rates and increased traffic capacity thereby meeting expectations of the rapidly growing mobile broadband. In a heterogeneous network 1, low power nodes (e.g. pico nodes) 3, 4 are placed throughout a high power node 2 (e.g. macro node) layout, providing pico cells within a macro cell. The macro node 2 and pico nodes 3, 4 can be deployed as separate cells (cells A, B and C in FIG. 1a) or as a shared cell (cell A in FIG. 1b). In the shared cell deployment, the same cell ID (e.g. PCI) is shared by a number of pico nodes 3, 4 and the macro node 2. The pico and macro nodes may also be denoted sectors.
There are several benefits with the shared cell deployment. A first benefit is the possibility of decoupling uplink reception and downlink transmission, which allows flexible network operation in that the reception points and transmission points may be selected independently of each other. This flexibility further provides a possibility of saving energy by muting some of the transmission points or reception points. Furthermore, in the shared cell concept not only uplink and downlink but also channels thereon can be completely decoupled, e.g. data channel and control channel transmitted on the downlink may be decoupled. It is possible to broadcast control channel in all sectors while selectively transmitting the data channel in some of the sectors; it is also possible to only transmit control information in macro to provide basic coverage, including system information, control and reference signals, while using pico to transmit data to enhance capacity and data rates.
Still further benefits of shared cell deployment comprise easy cell planning; since all sectors belong to the same cell, there is less concern about inter-cell interference. Further, there is some reduction in control signaling because there is no need to perform handover within the shared cell. Further, since it is possible to perform joint reception from different sectors (in uplink) and multi-point transmission (in downlink) in a shared cell, signal quality can be enhanced and thus higher spectrum efficiency can be achieved.
However, when combing different cells into one cell with a single PCI, the capacity will be limited by the cell specific physical resources, such as frequency bandwidth. Spatial Division Multiplexing (SDM) is introduced to improve capacity. In contrast to the basic LTE cell configuration, where all wireless devices camped in a cell share cell resources by time- and/or frequency multiplexing, there is yet another resource domain to use in a shared cell with SDM, namely spatial resource. Wireless devices that are spatially separated with good sector isolation can use the same time and frequency resource, but on different sectors.
In view of the scarce resources that are available in the shared cell concept, improvements in regard of the resource usage are sought for.